Method For Assessing The Mechanical Load Of An Individual

ABSTRACT

The invention deals with a method for assessing the mechanical load by an individual, said method comprising the following steps:—acquisition, over an analysis period, of three directional acceleration data using at least one acceleration measurement device carried by the individual,—transmission of the acceleration data thus acquired to a processing module and—determination by said module of at least one value assessing the mechanical load of said individual, characterized in that the processing module is adapted to implement on the acceleration data a calculation which result is independent of the spatial orientation of said device, said result assessing the mechanical load of said individual for a given time interval.

GENERAL TECHNICAL FIELD AND PRIOR ART

Professional sportsmen are commonly equipped with devices, during training and competitions, meant to acquire numerous data during their activity, notably to analyze their performances and follow their evolution.

Those data also provide bases which enable to adapt the training of sportsmen in function of their recorded performances, notably to maximize the benefits of the training while minimizing the chances of injury due to overtraining.

One of the most commonly used parameter to perform such an analysis is the PlayerLoad indicator.

The PlayerLoad provides a rating of the total mechanical charge (or load) applied to the sportsman, during his physical activity at a studied moment.

This indicator is based on the acceleration data acquired and recorded by an inertial unit commonly placed in the lumbar zone or between the scapulae (thought another location on the body might be considered, depending on the physical activity or movement which is measured). In some particular instances, said inertial unit can even be placed on the equipment worn by the subject or accompanying the subject in his movement, for example in the case of sporting equipment, bicycle, kayak etc. . . . .

Typically, the PlayerLoad is calculated for an instant T from the longitudinal acceleration component on the three axis X, Y and Z of the inertial unit, based on the following the formula:

PL_(T)=((a _(x,T) −a _(x,T−1))²+(a _(y,T) −a _(y,T−1))²+(a _(z,T) −a _(z,T−1))²)^(1/2)

The term a_(x,T) refers to the longitudinal acceleration on the axis X recorded at time T, as well as the terms a_(y,T) and a_(z,T) refer to the longitudinal acceleration on the respective axis Y and Z recorded at time T.

The terms a_(x,T−1), a_(y,T−1) ant a_(y,T−1) refer to the longitudinal accelerations on the respective axis X, Y and Z recorded at time T−1, which is the previous acquisition step of the central unit.

The acceleration vector at time T being expressed in the X, Y, Z basis as

$A_{T} = \begin{pmatrix} a_{x,T} \\ a_{y,T} \\ a_{z,T} \end{pmatrix}$

and the acceleration vector at time T−1 being

${A_{T - 1} = \begin{pmatrix} a_{x,{T - 1}} \\ a_{y,{T - 1}} \\ a_{z,{T - 1}} \end{pmatrix}},$

the PlayerLoad expressed as mentioned previously is at least proportional (in the present case similar) to the module of the time variation of the acceleration

${dA} = {\begin{pmatrix} {a_{x,T} - a_{x,{T - 1}}} \\ {a_{y,T} - a_{y,{T - 1}}} \\ {a_{z,T} - a_{z,{T - 1}}} \end{pmatrix}.}$

It is well known in the field of dynamics that the formula used to derivate a vector V expressed in a first base B1 mobile with respect to a second base B is the Bour's formula:

$\left( \frac{dV}{dT} \right)_{B} = {\left( \frac{dV}{dT} \right)_{B\; 1} + {\Omega_{B\; 1\text{/}B}\bigwedge V}}$

Wherein Ω_(B1/B) is the rotational speed vector of the base B1 with respect to the base B.

We can see that, our acceleration vector A being expressed in the base of the inertial unit which is secured to the sportsman's body, the base would be mobile with respect to a Galilean base.

As discovered by the inventors, the rating of the total mechanical load of a subject using the above-mentioned PlayerLoad is biased due to calculations with acceleration coordinates that are not in the same base. Consequently, the use of the PlayerLoad to evaluate or adapt a physical exercise would be far less effective as the PlayerLoad would suffer from a lack of accuracy.

With reference to FIGS. 1 a, 1 b, 1 c, and table 1 the bias associated to PlayerLoad when estimating mechanical load has been revealed thanks to a comparative test involving crossed measurement apparatus applied to sequences of characteristic movements performed by studied subjects. This is shown even more sensitive depending on the localization of the measurement device.

The measurement devices were on one hand a tri-dimensional accelerometer, worn by the subject, and on the other hand in series force plates on which the subject performed the movements and which are considered as a gold standard to measure center of mass displacements and therefore to provide objective measures of the work performed by the subject. The amount of mechanical work performed by the subject reflects the mechanical stress sustained by the subject.

The accelerations recorded for a movement by both devices were used to calculate the PlayerLoad as described above from measures obtained from each apparatus, those PlayerLoad values being displayed on FIGS. 1a to 1c using Bland-Altman plots.

The ordinates are the difference between the PlayerLoad calculated from the force plate and the PlayerLoad calculated from the accelerometer, the abscissas being the mean value between the PlayerLoad calculated from the force plate and the PlayerLoad calculated from the accelerometer.

FIG. 1a displays the PlayerLoad calculated when the subjects performed some general displacements. The intensity of the recorded data is quite low, but the bias observed between the two measures, which is a mean value of the difference between the apparatus data, is as high as 17.1, whereas the aimed result would be 0.

FIG. 1b displays the PlayerLoad calculated when the subjects performed a run acceleration. The mechanical load is higher than during the general movements, which results in a bias as high as 32.8 (see also table 1).

FIG. 1c displays the PlayerLoad calculated when the subjects performed a run at a constant velocity. The mechanical load is higher than during the run acceleration, which results in a bias as high as 59.4(see also table 1).

This shows that the more the effort is intense, and thus demanding for the sportsman, the less the PlayerLoad is accurate. This lack of accuracy diminishes greatly the efficiency of the use of the PlayerLoad in reporting the real mechanical load of the subject.

This should be taken into account in the calculation of the PlayerLoad in the foresaid formula. Moreover, one can expect this bias magnified if the PlayerLoad was to be used to express the quantity of mechanical stress supported for a long period due to the integration of this bias along the studied duration.

Method and apparatus disclosed in patent application EP1731097 aim at recognizing the activity of a subject independently of the orientation of the sensor unit, said method using a plurality of linear motion sensors.

GENERAL PRESENTATION OF THE INVENTION

A goal of the invention is to improve the reliability of mechanical load index deduced from longitudinal acceleration data.

Another goal of the invention is to optimize the use of such index in training and performance monitoring in sports.

Another goal of the invention is to increase the efficiency of training while decreasing the risk of injury.

According to one angle, the invention proposes a method for assessing the mechanical load of an individual, said method comprising the following steps:

-   -   acquisition, over an analysis period, of three directional         acceleration data using at least one acceleration measurement         device carried by the individual,     -   transmission of the acceleration data thus acquired to a         processing module and     -   determination by said module of at least one value assessing the         mechanical load of said individual,

characterized in that the processing module is adapted to implement on the acceleration data a calculation, result of which is independent of the spatial orientation of said device, said result assessing the mechanical load of said individual for a given time interval

According to another angle, the invention proposes a method for assessing the mechanical load of an individual, said method comprising the following steps:

-   -   acquisition, over an analysis period, of three directional         acceleration data using at least one acceleration measurement         device carried by the individual in the upper back zone,         preferentially between the scapulae,     -   transmission of the acceleration data thus acquired to a         processing module and     -   determination by said module of at least one value assessing the         mechanical load of said individual,

characterized in that the processing module is adapted to implement on the acceleration data a calculation of a scalar index, said scalar index comprising a term proportional to the absolute value of the difference of the acceleration norms determined at two different times, said scalar index being independent of the spatial orientation of said device and assessing the mechanical load of said individual for the given time interval.

Such a method is advantageously completed by the different following features taken alone or in combination:

-   -   The method further comprises a step of transmitting during which         the value thus calculated is sent to at least one interface         means, where it is displayed as an assessment of the physical         activity of the individual over said period of analysis;     -   the calculation of the scalar index is equal to the absolute         value of the difference between:         -   the square root of the sum of the square of the             accelerations along the different directions recorded at a             given time, and         -   the square root of the sum of the square of the             accelerations following the different directions recorded             during the preceding or following time;     -   during the acquisition step, angular velocity and/or relative         angles or rotations around the three axes of the space are         measured, the determination implemented by the processing module         being also a function of these measurements;     -   the method further comprises an integration step in which         several values assessing the mechanical load are integrated over         time;     -   the processing module or the interface module combines the value         of the mechanical load with at least one other relative datum to         the quantification of the physical activity of the individual         selected among physiological parameters, spatiotemporal         parameters of the displacement, subjective parameters of force         perception, parameters of global mechanics and/or parameters of         articular mechanics;     -   the values of mechanical loads in a specific time interval are         used to discriminate different phases of activity of said         subject;     -   the acceleration data are stored in a memory during the         acquisition step, the data being extracted only when the         analysis period is completed;     -   the at least one acceleration measurement device comprises a         single tridimensional accelerometer.     -   According to another angle, the invention proposes a program         comprising code elements configured to carry out the         implementation of the method for assessing the mechanical load         of an individual according to one of the preceding claims.     -   According to another angle, the invention proposes a device for         assessing the mechanical load of an individual comprising:     -   i) an acceleration data acquisition device making it possible to         detect accelerations in the three directions of the space,     -   ii) means for storing the recorded data,     -   iii) interface means with a user and     -   iv) means for implementing such a computer program.     -   According to another angle, the invention proposes such a method         of adapting the physical activity of an individual:     -   i/ implementing the method as exposed above,     -   ii/ comparing measurements of mechanical load of said individual         with corresponding values obtained at a different time,     -   iii/ adapting the physical activity of the individual according         to the result of the comparison of the previous step.

PRESENTATION OF THE DRAWINGS

Other features and advantages of the invention will come out of the description below, which is purely illustrative and not limitative, and shall be read with refer to the annotated drawings on which:

FIG. 1a is a graphic representing the agreement between the PlayerLoad calculated using the inertial unit and the PlayerLoad calculated using the force plate, having in ordinate the difference between the PlayerLoad of both devices and in abscissa the mean value between the PlayerLoad of both devices, the data being obtained during a sequence of general displacement;

FIG. 1b is a graphic representing the agreement between the PlayerLoad calculated using the inertial unit and using the force plate, having in ordinate the difference between the PlayerLoad from both devices and in abscissa the mean value between the PlayerLoad from both devices, the data being obtained during a sequence of run acceleration;

FIG. 1c is a graphic representing the agreement between the PlayerLoad calculated using the inertial unit and using the force plate, having in ordinate the difference between the PlayerLoad from both devices and in abscissa the mean value between the PlayerLoad from both devices, the data being obtained during a sequence of run at constant velocity;

FIG. 2 is a schematic representation of a measurement apparatus faithful to the invention equipped on a subject;

FIG. 3 is a schematic representation of a measurement process faithful to the invention;

FIG. 4a is a graphic representing the agreement between the mechanical load index faithful to the invention calculated using the inertial unit and using the force plate, having in ordinate the difference between the mechanical load index of both devices and in abscissa the mean value between the mechanical load index of both devices, the data being obtained during a sequence of general displacement;

FIG. 4b is a graphic representing the agreement between the mechanical load index faithful to the invention calculated using the inertial unit and using the force plate, having in ordinate the difference between the mechanical load index from both devices and in abscissa the mean value between the mechanical load index from both devices, the data being obtained during a sequence of run acceleration;

FIG. 4c is a graphic representing the correlation between the mechanical load index faithful to the invention calculated using the inertial unit and using the force plate, having in ordinate the difference between the mechanical load index from both devices and in abscissa the mean value between the mechanical load index from both devices, the data being obtained during a sequence of run at constant velocity.

DESCRIPTION OF ONE OR SEVERAL EMBODIMENTS

The embodiments described hereafter concern the case of a measurement process of the physical activity realized by an individual during a given period of time, and more precisely a sport player during a training or a competition. The purpose of this presentation is nonetheless purely illustrative and not limitative, as the measurement process could be applied also, for example, to monitor an individual in methods of the fields of medicine or physical rehabilitation.

With reference to FIG. 2, a physical activity measurement device 1 is secured to a sportsman's body. It may be placed in different places, notably in the lumbar zone, but preferably in the upper back zone, between the scapulae. Indeed, inventors have surprisingly found that the upper back unit provides better concurrent validity than the lower back mechanical load estimation, when compared to data generated by the force plate (Table 1). These findings are unexpected as one might preferably consider positioning the sensor unit position as near as possible of center of mass of the body i.e. near to the waist level, e.g. at the lower back. Inventors have found that placing the sensor unit at the lower back position leads to a large overestimate regarding the quantitation of the mechanical load underwent by a subject, especially while performing displacement at a high intensity level (table 1).

In a particular embodiment, the measurement device is placed in any other part of the body the movement of which is to be analyzed.

The physical activity measurement device 1 comprises at least one acquisition device 2 collecting the data.

In this embodiment, the acquisition device 2 is an inertial unit 3, configured to collect acceleration data on three different axes. In a particular embodiment, said device comprises a single tridimensional accelerometer, in a more particular embodiment said device is a single tridimensional accelerometer.

The acceleration data comprise three longitudinal acceleration components a_(x,T), a_(y,T), a_(z,T) on three different axes X, Y, Z of space, each sample being an acceleration vector A_(T) collected at an instant T. In others embodiments, the sample may comprise rotational speed data with respect to at least one of the three axes X, Y, Z.

A storing device 4 is configured to store the collected data during the acquisition period.

The storing device 4 can be secured to a main body 5 assembling the different parts of the acquisition device 2, or may be removed from the acquisition device 2, in which case a communication device 6 is configured to transfer the samples from the acquisition device 2 to the storing device 4.

Once the data are acquired and stored, a processing module 7 runs an analysis configured to convert the acceleration data into mechanical load index. The processing module 7 may comprise a computer, a chip, a tablet, or any dedicated device of the like.

A display device 8 is configured to communicate those data to a final user, enabling him to determine if, in function of the displayed data and his knowledges, adapting effort intensity of the subject is to be considered.

With reference to FIG. 3, the physical activity measuring device 1 performs a measuring process 9 comprising several steps.

The measuring process 9 of a physical activity comprises a step of acquisition 10, during which data are collected along a given period of time at a given sample rate.

In this embodiment, a sample collected at a moment T is an acceleration vector A_(T) comprising three longitudinal acceleration components a_(x,T), a_(y,T), a_(z,T) on three different axes X, Y, Z of space.

In another embodiment, the collected data could equally comprise rotational speed with refer to at least one of the three different axis X, Y, Z.

Once the measuring step 10 is finished, it starts over while starting the step of transfer 11.

The acquisition step 10 runs continuously during the studied period.

During the step of transfer 11, the sample A_(T) is sent to the storing device 4 where it is stored until the acquisition step 10 is finished.

In a different embodiment, data are sent directly and continuously to the processing module 7.

A step of determination 12 is then performed by the processing module 7, during which a mechanical load value is determined for each acceleration data set [A_(T), A_(T−1)].

In this embodiment, the value calculated from the acceleration data is a mechanical load index, which is at least proportional to the absolute value of the difference between:

-   -   the square root of the sum of the square of the different         components of a sample at an instant T,     -   the square root of the sum of the square of the different         components of a sample at an instant T−1,     -   following a formula such as:

mechanical load index=|(a _(x,T) ² +a _(y,T) ² +a _(z,T) ²)^(1/2)−(a _(x,T−1) ² +a _(y,T−1) ² +a _(z,T−1) ²)^(1/2)|

In this way, every sample would be associated with a mechanical load index value, which would give an estimation of the total mechanical load expressed by the sportsman at each instant T.

Of note, as shown in table 1 this index, alone, is surprisingly found sufficient to quantize the mechanical load of the individual with a good accuracy while minimizing bias when comparing the data provided by the force plates, even more when the sensor is placed at the upper black level, more particularly between scapulae.

During a communication step 13, those calculated data are sent to the display device 8, where they are available for the user.

The display device 8 may be a screen or the like.

In a particular embodiment, the processing module 7 is embedded in the main body 5 or more specifically in the acquisition device 2 and values of mechanical load index are stored in the storing device 4 until the extraction of the data to complete the determination step 12. In another particular embodiment, the processing module 7 is embedded in the main body 5 or more specifically in the acquisition device 2 and values of mechanical load index are sent directly and continuously by the communication device 6 to the display device 8 during the communication step 13.

In a particular embodiment, the processing module and/or the display device can combine the value of the mechanical load with at least one other datum relative to the quantification of the physical activity which are well known from the skilled in the art. It can be, for example, physiological data as heart rate, energy expenditure, or burned calories etc, spatiotemporal parameters as, distance, ascending or descending elevation or drop, speed, duration of physical activity etc. . . . . Others parameters like subjective parameters of force perception, parameters of global mechanics and/or parameters of articular mechanics as, for example, stride, pace, force, power or strength can also be used.

The wording “combine” is here to be understood as meaning any calculation in which various data are used, for example merged, to provide a new output data.

By way of example, those new output data can be index indicators which can easily be graphically displayed.

A further step of filter 14 can be applied, said filter step 14 being a low-pass filter in order to smoothen the obtained curve of Played Load data. A further step of integration 15 may be performed to qualify the total amount of energy spent during the studied period.

The mechanical load data may be time-integrated in order to provide an index to reflect the total amount of work realized by an individual during the studied period.

This index can provide a mean for comparing the different amounts of mechanical charge supported during trainings or competitions.

The reliability of this mechanical load index has been studied thanks to a «gold standard method», described above.

With reference to FIG. 4a , the data recorded when the subjects performed some general displacements provide a much more accurate mechanical load index, as the bias is now of only 0.6.

With reference to FIG. 4b , the data recorded while performing a run acceleration show a bias of only 0.9.

With reference to FIG. 4c , the data recorded while performing a run at constant velocity show a bias of only 5.4.

This invention can be profitably employed in the fields of inertial units, more precisely with onboard inertial units and particularly in medicine and sports. Inertial units are notably commonly used in devices such as mobile phones or connected watches to acquire and analyze data related to the movements performed by the user.

Mechanical load data may be recorded and studied over a long period of time, for instance a whole season of a sportsman, along with comparative performance data.

A comparative study can be performed after in order to identify the effects of the amount and the nature of each training program for each different sportsman according to their performances.

Those data could be used to produce a personalized training program which would produce the best performance enhancement while reducing the risks of injuries.

Enhancement of the performance may be obtained by calibrating the intensity and the type of the training program, for instance by increasing the intensity of exercises.

Reduction of the risks of injuries may be obtained by an analog calibration of the training program, for instance by avoiding the exercises that have been proven excessive, or decreasing the duration of exercises thus optimizing the recovering time. From another point of view, tracking physical activity of the subject through the method of the invention allows to setting up training sessions in a way to maintain a sufficient level of physical activity of the subject thereby in order to prevent injuries occurring upon intensive physical demand, as performed, for example, during sports competitions or the like.

It could also produce a training program configured to target particular physical points in order to practice on a weak spot or strengthen the qualities of the individual.

In fields such as rehabilitation or the medical monitoring of weakened patients, the mechanical load data may be used to spread the activity along the whole day in order to avoid peaks of physical stress.

The rehabilitation program would thus be more efficient due to a better monitoring of the health evolution in function of the exercises performed.

The mechanical charge data can also be used to detect a particular change in performances, and develop systemic models to relate those changes to injuries, thus enabling injury detection at the earliest, and then the adaptation of the training program and/or the triggering of a convenient treatment. This is of particular interest in the fields of high-level sports, where the worsening of an injury can preclude the sports season or even the whole carrier of the sportsman. 

1. A method for assessing the mechanical load of an individual, said method comprising the following steps: acquisition (10), over an analysis period, of three directional (X, Y, Z) acceleration data using at least one acceleration measurement device (2) carried by the individual in the upper back zone, preferentially between the scapulae, transmission (11) of the acceleration data thus acquired to a processing module (7) and determination (12) by said module (7) of at least one value assessing the mechanical load of said individual, characterized in that the processing module (7) is adapted to implement on the acceleration data a calculation of a scalar index, said scalar index comprising a term proportional to the absolute value of the difference of the acceleration norms determined at two different times, said scalar index being independent of the spatial orientation of said device (2), said scalar index assessing the mechanical load of said individual for the given time interval.
 2. The method according to claim 1, further comprising a step of transmitting (13) the value thus calculated to at least one interface means (8), where it is displayed as an assessment of the physical activity of the individual over said period of analysis.
 3. The method according to claim 1, characterized in that the calculation of the scalar index is equal to the absolute value of the difference between: the square root of the sum of the square of the accelerations along the different directions recorded at a given time, and the square root of the sum of the square of the accelerations following the different directions recorded during the preceding or following time.
 4. The method according to claim 1, wherein during the acquisition step (10) angular velocity measurements and/or relative angles or rotations around the three axes of the space are measured, the determination implemented by the processing module being also a function of these measurements.
 5. The method according to claim 1, further comprising an integration step (15) in which several values assessing the mechanical load are integrated over time.
 6. The method according to claim 1, wherein the processing module (7) or the interface module (8) combines the value of the mechanical load with at least one other relative datum to the quantification of the physical activity of the individual selected among physiological parameters, spatiotemporal parameters of the displacement, subjective parameters of force perception, parameters of global mechanics and/or parameters of articular mechanics.
 7. The method according to claim 1, characterized in that it uses, on values of the mechanical load successively determined in time, one or more different phases of activity.
 8. The method according to claim 1, wherein the acceleration data are stored in a memory (4) during the acquisition step (10), the data being extracted during the implementation of the estimation method performed when the acquisition step (10) is completed.
 9. The method according to claim 1, wherein that the at least one acceleration measurement device (2) comprises a single tridimensional accelerometer.
 10. Computer program comprising code elements configured to carry out the implementation of the method for assessing the mechanical load of an individual according to one of the claims 1 to
 9. 11. Device for assessing the mechanical load of an individual comprising i) an acceleration data acquisition device (2) making it possible to detect accelerations in the three directions (X, Y, Z) of the space, ii) means for storing (4) the recorded data, iii) interface means (8) with a user and iv) means for implementing the computer program according to claim
 10. 12. A method of adapting the physical activity of an individual, comprising the steps of: i/ implementing the method according to any one of claims 1 to 9, ii/ comparing measurements of mechanical load of said individual with corresponding values obtained at a different time, and iii/ adapting the physical activity of the individual according to the result of the comparison of the previous step.
 13. The method according to claim 12, in which the comparison of different measurement values according to step ii) shows an overactivity of the individual, adapting the physical activity according to step iii) then consisting in a decrease in the level of physical activity of said individual to prevent injuries or to reduce the training load.
 14. The method according to claim 12, wherein the comparison of different physical activity measurement values according to step ii) shows a defect in the physical activity of the individual, the adaptation of the physical activity according to step iii) then consisting of an increase in the level of physical activity of said individual to optimize the effects of physical activity. 